Radio Frequency Identification (RFID) systems are commonly used to locate and track items in a near-field communication network including a reader device and at least one wireless terminal, or tag. Energized time-varying electromagnetic radio frequency (RF) waves, which comprise the carrier signal, are transmitted from the reader to the tags in a given RFID network or system. Tags use backscatter technology to reflect the reader's RF signal back to the reader, modulating the signal to encode and transmit data.
FIG. 1 depicts a prior art RFID system in which data transmission from tags 101a-c to reader device 103 is performed on a same frequency channel or spectrum 104. Using the established backscattering technology, each of the plurality of tags typically in the RFID system or network sends RF signals on the same backscattered carrier signal. Hence, the backscattered RF signals from each tag overlap those of other tags within the same RF spectrum associated with that given reader device/RFID network.
As a consequence, tag collision in RFID systems occur when the multiple tags are energized by the same RFID reader device, and simultaneously reflect their respective, overlapping signals back to the reader using the given frequency channel. Thus the tag collision problem is exacerbated whenever a large number of tags must be read together in the same RF field. The reader is unable to differentiate these signals when the simultaneously generated signals collide. The tag collisions confuse the reader, generate data transmission errors, and generally reduce data throughput within the RFID system or network.
Various systems have been proposed to isolate individual tags. For example, in one technique aimed at reducing collision errors, when the reader recognizes that tag collision has taken place, it sends a special “gap pulse” signal. Upon receiving this signal, each tag consults a random number counter to determine the interval to wait before sending its data. Since each tag gets a unique number interval, the tags send their data at different times. The adverse impact on overall RFID system performance, in terms of data throughput rate, however, still exists.
Modulating the signal received by the tag and re-radiating the modulated signal backscattered to the reader device is known, using such signal modulation schemes, such as phase shift keying (PSK) and amplitude shift keying (ASK), where the tag changes its reflection coefficient by changing the impedance match between states. However, the adverse effects of tag collisions resulting from overlapping backscattered signals on a given frequency channel still remain.
Moreover, especially pertinent in the context of high frequency RF signals is the effect of the DC offset in the reader device and the effects of the reader's phase noise.
The design of backscattering tag terminals in RFID networks involves some further special challenges. The backscattered tag signal is not the only reflected signal present; in a single-antenna system there is usually an even larger signal due to unintended reflections from the transmitting antenna of the reader device to various surrounding objects. The unwanted reflected signals mix with the local oscillator signal in the reader device; since they are not (usually) modulated they produce DC offsets: large DC voltages output from the mixer. Fortunately, if the wanted signal does not contain much information near DC, which will be the case as long as the tag symbols are chosen to ensure frequent transitions in tag state even when the data has long strings of 1's or 0's, it may straightforward to filter out this offset. However, this is generally not preferred. The resulting large swings in the mixer output are harder to filter out, and make it hard to see any reflection from the tag until the receiver has had a few microseconds to recover.
The above problem may be ameliorated by using separate transmit and receive antennas (a bistatic configuration): in this case instead of the reflected signal from the (single) antenna, the receiver must only deal with the portion of the transmitted signal that impinges on the receive antenna, typically much smaller than the reflected signal. Isolation of around 40 dB is obtained with the large bistatic antennas commonly used with commercial readers, which represents 20-25 dB better than the return loss from a single antenna. However, the use of a pair of antennas adds cost, complexity, and increased space requirements to the reader. An adaptive antenna tuner or nuller to reduce the reflections from a single (monostatic) antenna may alternatively be used, but this solution again involves added expense and complexity.
Oscillators, for instance as used in a reader device, do not produce a perfectly pure carrier signal, as both the phase and amplitude of the signal can vary. The phase noise can be converted into amplitude noise in the received signal when the large fixed reflection mixes with the local oscillator. Phase noise is normally highest at frequencies very close to the frequency of the carrier signal, which is converted to near DC upon mixing. To reduce phase noise effects, it is desirable to use a relatively narrow filter that passes only the frequencies containing the wanted signal from the tags, and to use the highest, or band-pass, tag modulating frequency as practical; however, this restricts the types of modulations and the type of data streams that may be used.